ipv4: Fix crashes in fib_rules_tclass().

All paths assume, when CONFIG_IP_MULTIPLE_TABLES is enabled, that any
successful call to fib_lookup() will initialize the fib_result->r
value to something.

We violated that expectation in the new fib_lookup() fast path.

Reported-by: Or Gerlitz <ogerlitz@mellanox.com>
Tested-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -230,6 +230,7 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 			     struct fib_result *res)
 {
 	if (!net->ipv4.fib_has_custom_rules) {
+		res->r = NULL;
 		if (net->ipv4.fib_local &&
 		    !fib_table_lookup(net->ipv4.fib_local, flp, res,
 				      FIB_LOOKUP_NOREF))